The present invention relates to a chain-configured interface bus system and a communications arrangement for use therein.
Installations are known in which clusters of data transmitting and receiving devices are inter-connected by an interface bus sytem that comprises a respective sub-bus system associated with each cluster of devices, and communication links connecting the sub-bus systems into a chain. Generally, a system controller is connected to one of the chain-end sub-bus sytems (herein designated the head or up-chain end of the the system) and serves to set up the installation for the transfer of data between the devices via the interface bus system. Each sub-bus system takes the form of a sub-bus to which the devices of the associated cluster are connected, and a communication arrangement for transferring data over the links between the sub-bus system and its up-chain and down-chain neighbours. The communications arrangement comprises an up-chain bus extender for interfacing the sub-bus with the up-chain link (where present) and a down-chain bus extender for interfacing the sub-bus with the down-chain link (also where present).
During data transfer from a currently-selected data-transmitting device (source) to the or each currently-selected data-receiving device (acceptor), each sub-bus system can be considered as having one source and one or more acceptors. The sources of the sub-bus systems that do not include the original data-transmitting device are constituted by the bus extenders that receive the data from the originating sub-bus system over the communication links of the chain; the bus extenders that pass on the data from their own sub-bus to a neighbouring system act as acceptors in their sub-bus systems.
The transfer of data between devices is usually governed by a handshake procedure between the current source and acceptors. One widely used handshake procedure is that forming part of the IEEE-488 protocol standard and described in U.S. Reissue Pat. No. 29,246 (assignee Hewlett-Packard Company). With an installation including a plurality of sub-systems connected into a chain, it is possible to implement the handshake procedure independently in each sub-bus system. However, for various reasons, such an independence of the handshake procedures on the sub-bus systems is not entirely satisfactory. As a result, an alternative procedure has found acceptance in which the bus extenders are effectively transparent and the handshake procedure carried out by the original data-transmitting device involves the participation of all current acceptors regardless of their sub-bus system association. This is achieved by arranging for the `acceptor` bus extender on the original source's sub-bus system to delay its handshake responses with the source until it has received appropriate responses from the neighbouring sub-bus system extender to which it is linked indicating that the handshake procedure in which this extender is involved as a source, has progressed to an appropriate stage. Thus the `acceptor` bus extender on the originating sub-system may delay indicating data acceptance until all the acceptors of the neighbouring sub-system have indicated their acceptance to the source bus extender of that sub-system and that extender has communicated back this acceptance to the acceptor bus extender of the originating sub-bus system. Of course, one of the acceptors of the neighbouring system may itself be a bus extender in which case the indication of data acceptance by this neighbouring system will be delayed until the sub-bus system fed by its acceptor bus extender has indicated data acceptance, and so on along the chain of sub-bus systems.
A description of one arrangement for inter-relating the handshake procedures on neighbouring sub-bus systems to achieve the foregoing inter-dependence is given for the IEEE-488 protocol handshake procedure in U.S. Pat. No. 4,451,886 (assignee Hewlett-Packard Ltd).
Although the purpose and effect of rendering the handshake procedures on the sub-bus systems inter-dependent, is to overcome certain disadvantages exhibited by independent-handshake schemes, a drawback exists in that if a sub-bus system should fail to respond to handshake related signals from its neighbouring sub-bus system, then the handshake procedure in this latter system is frozen which, in turn, freezes the whole system. One reason why a sub-bus system might fail to respond is that its local power supply has been switched off by mistake, or the communicating link has been broken.
It is an object of the present invention to provide a chain-configured interface bus system which while exhibiting inter-dependence of sub-bus systems, is capable of continued but reduced operation in the presence of a non-responsive sub-bus system.